1. Field of the Invention
This invention relates to a neutral polarizer which provides linearly polarized light with excellent tone and is useful to improve visibility of a liquid crystal display, and the like.
The present application is based on Japanese Patent Application No. Hei. 11-117549 and 2000-73212, which are incorporated herein by reference.
2. Description of the Related Art
Dichroic polarizers having a dichroic substance, such as iodine, Herapathit, or a dichroic dye, adsorbed and orientated on a substrate are known. These polarizers are to obtain linearly polarized light by making use of light absorption by the dichroic substance and have been frequently used in liquid crystal displays, etc. However, the degree of polarization is dependent on wavelength, showing some scatter in the visible region. In general, the polarization degree of polarized light having a wavelength around 400 nm, which is close to the ultraviolet region, tends to be low.
The wavelength dependence of polarization degree causes leakage of light when the dichroic polarizers are arranged in a crossed-Nicol. It follows that, where the dichroic polarizers in a crossed-Nicol are used in a liquid crystal display to make a dark image, the black screen gets blue-tinged, or a full-color image suffers from color mixing due to blue deficiency in black areas, resulting in reduced color reproducibility. Where the concentration of adsorbed dichroic substance that absorbs light in the shorter wavelength region is increased to increase the light absorption thereby to reduce light leaks, the white display image is yellowed because shorter wavelength light is absorbed more than longer wavelength light, also resulting in reduction of color reproducibility.
Accordingly, an object of the present invention is to provide a polarizer which provides transmitted polarized light whose polarization degree shows reduced wavelength dependence.
Another object of the present invention is to provide a liquid crystal display having minimized color deficiency and excellent achromaticity and therefore exhibits improved visibility with excellent color reproducibility in both a black and a white display mode.
The present invention provides a neutral polarizer which comprises a dichroic polarizing sheet and a polarizing and scattering sheet which has anisotropy in light scattering properties depending on the direction of polarization in such a manner as to have an optical axis in which linearly polarized light is scattered most intensely and that the scattering intensity of the linearly polarized light of that optical axis shows strong wavelength dependence, the absorption axis of the dichroic polarizing sheet and the optical axis of the polarizing and scattering sheet in which linearly polarized light is scattered most intensely being in parallel to each other.
The present invention also provides a liquid crystal display having the above-described neutral polarizer as a polarizer on the light source side of a liquid crystal display panel.
In the neutral polarizer according to the present invention, the optical axis of the polarizing and scattering sheet in which linearly polarized light is transmitted efficiently (the direction in which linearly polarized light is hardly scattered) and the transmission axis of the dichroic polarizing sheet agree with each other. As a result, the neutral polarizer exhibits excellent transmitting properties in that axial direction to satisfactorily maintains the hue ascribed to the dichroic polarizing sheet. Further, in the optic axial direction of the polarizing and scattering sheet which agrees with the absorption axis of the dichroic polarizing sheet, linearly polarized light is scattered intensely, and the intensity of the scattering in this direction has great dependence on wavelength, so that the transmittance for light, particularly in the shorter wavelength side, is reduced. As a result, leakage of light in the shorter wavelength side attributed to the dichroic polarizing sheet can be suppressed thereby to make the light achromatic, whereby the polarized light in both of the above-described optic axes can be made achromatic.
Thus, polarized light having been transmitted through the neutral polarizer has reduced wavelength dependence of polarization degree. It follows that a crossed-Nicol of the neutral polarizers hardly causes light leakage, the image in a black display mode is prevented from getting chromatic, color mixing due to color deficiency in a full-color display mode is prevented, and yellowing of a white image due to the difference in absorption dependent on wavelength is prevented. As a result, there are provided a liquid crystal display and the like which have satisfactory color reproducibility and satisfactory visibility. Where the neutral polarizer is disposed on the light source side of a liquid crystal display panel, hindrance to visibility attributed to back scatter by the polarizing and scattering sheet can be avoided thereby securing the improvement on visibility, such as contrast. Additionally, the neutral polarizer of the invention can be produced efficiently simply by superposing the constituent layers on each other and is suited to mass production.